Due to the possibility of the boss's attack patterns having changed, Asuna commanded her party members, including the one-handed sword-wielding Nautilus and the spear-wielding Sanza, to split into smaller groups. Sanza's spear was not durable enough to guard against the boss's wide-range attacks, thus Nautilus was expected to defend him; however, Nautilus failed to comply with Asuna's orders, leaving Sanza defenceless.

Before Sanza could retreat, he suffered a wide-range attack from the boss, which resulted in his weapon being broken and Sanza himself crashing into the wall, temporarily stunned. As the boss possessed an algorithm known as «Bullying», it immediately ignored its previous targets and focused its next attack on the collapsed spear user. Just before Sanza could suffer a finishing blow, the attack was intercepted by Kirito.

Due to Kirito joining the battle, the boss was easily defeated. After a short chat, Kirito proceeded further into the Labyrinth, while Asuna focused on investigating why Nautilus had ignored her order. As Nautilus had seemingly not suffered any damage or debuffs, Asuna requested him to explain the situation. The one-handed sword user reluctantly admitted that his legs had refused to move forward, despite his attempts to do so. After a moment of deliberation, Asuna commanded everyone into formation to depart from the Labyrinth.

Having reached the main town of the 40th Floor, Asuna disbanded the party and used the Teleport Gate to travel to Nolfret, the main town of the 39th Floor, where the Knights of the Blood headquarters were located. At the rural mansion that served as the headquarters, she headed to Heathcliff's room, where she reported on the daily clearing efforts to the guild leader. Heathcliff became interested when Asuna mentioned that there had been an issue during the mid-boss battle, and thus request further details on the matter.

Having learnt of Nautilus's inability to fight during the boss battle, Heathcliff hypothesised that the boy might be suffering from a slight case of «FullDive Non-Conformity». In Natilus's case, Heathcliff believed that the NerveGear was giving priority to the commands of his lower cerebral centres, the origin of instincts, instead of the higher centres, responsible for reason, thus making Nautilus's avatar unresponsive when at times of perceived danger. Although the guild leader doubted that the issue would be resolved entirely by mere training, due to Asuna's evaluation that the boy had potential, he entrusted the sub-leader with the final decision on his fate. After some hesitation, the girl declared that she would not allow Nautilus to participate in the next boss battle but would work on a way to control the condition after the battle.

Following the disbandment of his party, Nautilus wandered around Jaileum, the main town of the 40th Floor, until he eventually rented a room at a cheap inn to avoid contact with his guildmates. There, he lamented his blunder during the mid-boss battle by pointlessly attempting to stab himself with a dagger, only for it to be deflected by the Inner Area barrier, until the dagger fell out of his hand. As he bemoaned how his condition made him unable to fulfil his promise of rescuing his childhood friend from the death game, Nautilus received a message from the only person he had registered as a friend and left the room moments after reading the letter he had received.

At the plaza of Jaileum, Nautilus met with Yuna, his childhood friend and the only person he was friends with in the game. Having immediately noticed her friend's depression, Yuna decided to comfort him by giving him a piece of her candy and singing to him next to an NPC orchestra. Her song attracted a large audience, thus, as soon as she finished the song, Yuna escaped from the plaza, dragging Nautilus by his arm, in embarrassment. While having a conversation about Yuna's singing with her, Nautilus noticed that a new buff icon had lit under his Hit Point bar. Upon the boy realising that the buff came from her song, Yuna revealed that the buff was the effect of her Extra Skill named «Chant» and shared with him her desire to become part of the Clearers.

Having awoken at around 01:00, Asuna left Nolfret for a hilly area outside town. There, she encountered a «Balloon Roussette» bat, which unexpectedly ignored her and flew in the opposite direction. By following the bat, Asuna found Kirito in battle against a group of Balloon Rousettes, making use of their ability to attract others of their kind for farming efficiency. Just as Asuna began pondering whether she should return another day, the man in black disengaged from combat and approached her, suggesting to relocate before they started a conversation. Thus, the two travelled to a small spring at the edge of the hilly area, avoiding detection by any monsters.

After thanking Kirito for his help earlier that day and explaining how she used information from Argo to deduce where he would be conducting his farming that night, Asuna requested for his advice on Nautilus's case. Following a long period of silence, Kirito concluded that Heathcliff's assessment was most likely correct and thus Nautilus should avoid fighting on the front lines until the problem was resolved. As a last resort, Asuna wondered whether change of build could help overcome the problem, mentioning Nezha as an example, but Kirito stated that Nautilus's condition was too severe for this to be effective. With Kirito having affirmed her own conclusion, Asuna thanked the man in black and, once his Color Cursor vanished from Asuna’s vision, went towards town.

Having reached the town at around 02:00, Asuna decided to go for a walk, instead of returning to bed. During her walk, she spotted a gathering of five or six people in the plaza, despite the late time and the floor being unpopular. To her surprise, Asuna noticed an unusual sight - the group was listening to an NPC orchestra, with a female player singing. Before Asuna could speak to her, the girl used the nearby Teleport Gate to leave the scene, thus Asuna turned to the audience to enquire about the girl. The players explained that they did not know the singer's name but shared that the girl would come to sing at a random floor late at night each day, her songs giving a buff to any listener. Asuna deduced that the player had the «Chant» Extra Skill and wished for a chance to listen to the mysterious singer again with Kirito.

Later that day, the KoB clearing party encountered Kirito at the topmost part of the Labyrinth and reached the boss room together. After returning to Nolfret and dissolving her party, Asuna followed Kirito to the 36th Floor. There, she found him listening to the mysterious singer at the edge of a large crowd and decided to join him in listening to the song.

That same day, having been informed of his exclusion from the 40th Floor boss battle, Nautilus wandered around town until night, when he secluded himself in a cheap inn. Two days later, Nautilus visited the plaza to watch as the raid party departed to the Labyrinth through a Corridor Crystal. Once the last member of the raid entered through the portal, Nautilus was about to leave the plaza, when he was suddenly approached by Yuna, whom he had been avoiding due to his depressed state. Noticing the reason for Nautilus's poor mood, Yuna invited him for a meal.

While chatting at a cafe, Nautilus and Yuna suddenly heard a cry for help coming from the western gate. At that moment, the two noticed a wounded man with a curved sword running through the gate. Desperate, the man shouted that his comrades had been imprisoned in a field dungeon, where they were assailed by an army of monsters. As the monsters had inflicted a «Silencing» debuff on the party, rendering escape by Teleport Crystals impossible, Nautilus decided to quickly form a rescue party to save the trapped players. Having gathered seven or eight players, including two members from Fuurinkazan, for the cause, he requested Yuna to wait for him in town, but to his surprise, Yuna conclusively stated her desire to join the party as well and equipped her battle equipment. As Yuna's resolve was too firm to change her mind, Nautilus could not stop her from joining the rescue party on their journey to the dungeon.

At the dungeon, the group found five players in battle against five infinitely respawning monsters in a prison cell-like room, with the dungeon's boss lurking in wait deeper in the room. Upon enquiring about the location of the device for opening the cell, Nautilus learnt that the lever was located behind the boss. As it would be impossible to evacuate everyone from the room safely before it was sealed again, especially with the boss joining the battle, Nautilus concluded that the best method of rescuing the trapped players was to defeat the boss. As the other members began hesitating, Yuna used one of her songs to boost morale and grant all the players in the area a regeneration buff. Upon finishing the song, Yuna ordered the trapped party to unlock the doors by activating the lever behind the boss.

Upon entering the room, the rescue party focused on fighting the boss, the official name of which was Feral Warder Chief, while curved sword user's party was ordered to distract the boss's henchmen. The battle proceeded flawlessly for eight minutes until the boss's last hit point bar turned orange. At that point, a member from Fuurinkazan became concerned that the boss's attack patterns might change upon the hit point bar turning red; however, Nautilus, assuming that the Feral Warder Chief would act the same as the Ruthless Warder Chief, assured his party that the boss would not undergo any change in pattern upon its hit point bar turning red.

Contrary to Nautilus's expectations, moments after the boss's hit point bar reached the red zone, fifteen extra henchmen entered the room through holes in the walls. Just as the players were distracted by this unexpected development, the boss unleashed an unexpected area of effect attack, paralyzing most of the players, except for Nautilus, the two Fuurinkazan members, and Yuna, in its vicinity. As the players dealing with the henchmen became overwhelmed and inflicted with Silencing, Nautilus considered using crystals to at least escape with Yuna.

However, predicting Nautilus's intentions, the girl pleaded the boy to rescue the others, before using one of her songs to draw all the monsters to herself. Just as Nautilus was about to charge to Yuna's aid, a sudden episode of FullDive Non-Conformity lead to his avatar ignoring any of his commands, leaving him helpless to watch as his childhood friend died. At that very moment, the two Fuurinkazan players managed to defeat the boss, saving the other players. Later that same day, Nautilus withdrew from the guild and disappeared from everyone's sight.

Two years and seven months later, upon Eiji awaking from a dream of the past and his Augma device rebooting, YUNA the Artificial Intelligence (AI) appeared in the room, holding a bottle of pieces of candy, on her lap. YUNA was unaware that the candy represented the memories stolen from Sword Art Online players which Eiji and Professor Shigemura Tetsuhiro planned to use to revive the real Yuna as an AI once the bottle was full. Eiji had resorted to forceful measures, including physical injury, to gather them, not minding going to prison for his actions if it meant his childhood friend could return. As he contemplated on this, Eiji noticed that the sky looked like it did on the day of Yuna's death.